Abstract

A quasi-coherent edge density mode with frequency fmode ∼ 40 kHz is observed in Ohmic plasmas in National Spherical Torus Experiment using the gas puff imaging diagnostic. This mode is located predominantly just inside the separatrix, with a maximum fluctuation amplitude significantly higher than that of the broadband turbulence in the same frequency range. The quasi-coherent mode has a poloidal wavelength λpol ∼ 16 cm and a poloidal phase velocity of Vpol ∼ 4.9 ± 0.3 km s−1 in the electron diamagnetic direction, which are similar to the characteristics expected from a linear drift-wave-like mode in the edge. This is the first observation of a quasi-coherent edge mode in an Ohmic diverted tokamak, and so may be useful for validating tokamak edge turbulence codes.

Support and contributions from N. Crocker, E. Fredrickson, S. Kaye, S. Kubota, B. LeBlanc, R. Maingi, R. Maqueda, T. Munsat, S. Sabbagh, Y. Sechrest, J. R. Myra, D. A. Russell, and the National Spherical Torus Experiment Team are gratefully acknowledged. One of the authors (S.B.) would also like to thank H. Zushi and J. Ghosh for many useful discussions during the course of this work. This work was supported by U.S. DOE Contract DE-AC02-09CH11466. The digital data for this paper can be found at http://arks.princeton.edu/ark:/88435/dsp0170795b055.